The present invention relates to computer numerical control (CNC) machines and more particularly, to a method and apparatus for generating commands for controlling a CNC machine.
Computer numerical control (CNC) machines are utilized to automatically fabricate machined parts and molds. Based on computer generated commands, CNC machines move one or more cutting tools in three-dimensional space to mill, cut, bore and otherwise shape a work piece into a desired form. Shaping the work piece into the desired form is typically a process in which each tool is moved in a series of paths to gradually cut away material of a workpiece until the work piece takes on the desired form with the desired finish.
Computer Aided Manufacturing (CAM) software provides for substantially automating the process of creating the computer commands used for moving the cutting tools of a CNC machine. Generally, CAM software, accepts two-dimensional and three-dimensional design information which defines the dimensions of the workpiece from which the part is to be machined and also defines the dimensions of the finished part. The design information may be manually input by a user and also input from data files produced by computer-aided design (CAD) software. Based on the design information, CAM software generates a series of tool paths for each tool of a set of cutting tools selected by the user to produce the machined part in the desired form. CAM software also typically suggests to the user, cutting parameters such as tool feed rates and spindle speeds, in addition to computing the tool paths, although it is common to provide for user overrides of the suggested feed rate settings and spindle speeds.
Of great importance to CNC machining is: (1) the speed in which the computer commands can be created from the design information and (2) the speed at which a given part may be machined. Known CAM software includes numerous features designed to improve the speed of the CNC machine cutting process. However, the features for improving the speed of machining generally come at the expense of increased time to create the computer commands. Such features include gouge avoidance, edge protection, scallop-height control, gouge checking, isolation of steep and shallow machining, integrated tool path verification and remaining stock machining, all of which features are well-known in the field.
Known CAM software used for generating the tool paths for complex milling such as that required for machining molds, generally generates CNC commands that result in three phases of machining to form the complex machined part, i.e.: (1) rough milling, (2) intermediate milling, and (3) finish milling.
The rough milling process frequently consists of milling away parallel, planar slices of the work piece at a succession of constant Z levels. The process is known generally as xe2x80x9cZ level roughingxe2x80x9d or xe2x80x9cconstant Z millingxe2x80x9d. The Z level roughing process typically uses a relatively large (roughing) tool or tools to remove a large amount of material relatively quickly, but it also leaves a series of xe2x80x9csteps,xe2x80x9d or xe2x80x9cterraces,xe2x80x9d on any surfaces that are non-vertical or non-horizontal. The height of the steps formed by a single tool in the Z level roughing process is typically a constant value, the height of the steps being a function of a predetermined parameter which determines a depth of cut of the roughing tool. The larger is the depth of cut of the roughing tool, the higher are the resulting steps.
In contrast to the height of each step being held constant, the xe2x80x9cwidthxe2x80x9d of each step varies as a function of the diameter of the tool, the topology of the tool, the depth of the cut, and the topology of the surface being machined. Accordingly, the width of a step can vary greatly over a curved surface. As the surface approaches vertical, the width of the steps gets narrower. As the surface approaches horizontal, the steps get wider.
The relatively large size of the roughing tool generally precludes the CNC machine from cutting a smooth curved surface into the material of the workpiece. It is also extremely common that, in all but the simplest cases, the roughing tool is not able to mill detailed portions of a given planar slice. However, finish milling requires that the load on the finish tool be kept within an acceptable range in order to produce the required finish, necessitating a generally uniform workpiece surface be left by the roughing process. Consequently, before finish milling can begin: (1) the height and the width of the steps produced in rough milling must be reduced to acceptably small dimensions, and (2) the detailed areas in each slice, where the large tool is unable to cut due to its relatively large diameter, must be machined out. Accordingly, an intermediate milling process is generally performed by CNC machines between the rough milling and the finish milling phases of machining the workpiece.
Known techniques for preparing the workpiece for finish milling, i.e. intermediate milling, typically employ a process referred to as remaining stock (REST) milling. REST milling is a process for identifying the remaining material left over after a previously made rough milling cut, and removing the remaining stock using a tool or tools which are generally smaller than the rough cutting tool. REST milling generally comprises the following steps: (1) the difference between the final work piece shape and the shape of the cut made by the existing tool is computed; (2) a new smaller tool is generally chosen by the user; (3) a tool path is computed for the new tool in order to cut the remaining material in an efficient manner; (4) the remaining material is cut with the new tool; and (5) the process is repeated with successively smaller and smaller tools until the desired surface uniformity is achieved.
Known REST milling methods suffer from certain drawbacks and inefficiencies. In particular, typical REST milling requires the computation of the three dimensional volume of material that remains after each tool is finished milling the workpiece. The volume computation is required in order to program the motion of the next tool to avoid either xe2x80x9ccutting airxe2x80x9d or overloading the tool with too heavy a cut. The computation of the three dimensional volume results in a substantial computational load, significantly increasing the time required to generate the CAM software program output. Further, for complex design shapes, the shape of the initial tool path for the larger rough cutting tool frequently leaves significant uncut portions of material. Using a substantially smaller tool for each successive REST milling step results in slower material removal rates, due to the reduced depth of cut and step over for the smaller tool. Alternatively, if the same size tool is used for successive REST milling steps, significantly greater programming time is required. Choosing an intermediate size REST tool to maximize the tool removal rate often results in the need to perform additional iterations with a still smaller tool.
There is a need for a CAM software program which produces a series of tool paths which result in a machined surface which is ready for finish milling, and which: (1) reduces the amount of human-interaction time required for programming the CAM software program, (2) is faster to create the tool paths than existing CAM software programs and (3) creates tool paths which result in faster machining of a complex machined part than existing CAM software programs.
The present invention comprises an automated computer-implemented method, apparatus and article of manufacture for generating commands for controlling a computer numerical control machine to fabricate an object from a workpiece. The method includes the steps of: (1) determining a first set of Z coordinates for machining a first set of Z level planar slices with a first tool; and (2) determining a second set of Z coordinates for machining a second set of Z level planar slices with the first tool. The second set of Z coordinates is partitioned into one or more subsets. Each subset corresponds to a pair of adjacent Z coordinates belonging to the first set of Z coordinates. A distance between the Z coordinates of each subset is a unit fraction of a distance between the Z coordinates of the pair of adjacent Z coordinates which corresponds to each subset.